Using the GNU Privacy Guard: Agent PKSIGN

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#### 2.6.2 Signing a Hash

The client ask the agent to sign a given hash value. A default key will be chosen if no key has been set. To set a key a client first uses:

       SIGKEY <keyGrip>

This can be used multiple times to create multiple signature, the list of keys is reset with the next PKSIGN command or a RESET. The server test whether the key is a valid key to sign something and responds with okay.

       SETHASH --hash=<name>|<algo> <hexstring>

The client can use this command to tell the server about the data <hexstring> (which usually is a hash) to be signed. <algo> is the decimal encoded hash algorithm number as used by Libgcrypt. Either <algo> or –hash=<name> must be given. Valid names for <name> are:

`sha1`
`sha256`
`rmd160`
`md5`
`tls-md5sha1`

The actual signing is done using

       PKSIGN <options>

Options are not yet defined, but my later be used to choose among different algorithms. The agent does then some checks, asks for the passphrase and as a result the server returns the signature as an SPKI like S-expression in "D" lines:

         (sig-val
           (<algo>
             (<param_name1> <mpi>)
     	   ...
             (<param_namen> <mpi>)))

The operation is affected by the option

       OPTION use-cache-for-signing=0|1

The default of `1` uses the cache. Setting this option to `0` will lead `gpg-agent` to ignore the passphrase cache. Note, that there is also a global command line option for `gpg-agent` to globally disable the caching.

Here is an example session:

       C: SIGKEY <keyGrip>
       S: OK key available
       C: SIGKEY <keyGrip>
       S: OK key available
       C: PKSIGN
       S: # I did ask the user whether he really wants to sign
       S: # I did ask the user for the passphrase
       S: INQUIRE HASHVAL
       C: D ABCDEF012345678901234
       C: END
       S: # signature follows
       S: D (sig-val rsa (s 45435453654612121212))
       S: OK

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